Sept. 1: Romney\'s Convention Bounce Appears Middling So Far

September 1, 2012, 7:47 pm

By NATE SILVER

The Republican National Convention in Tampa, Fla. received mediocre television ratings - and the polling data so far suggests that it may produce only a modest bounce in the polls for Mitt Romney.

The most favorable number for Mr. Romney is from the Rasmussen Reports tracking poll. That survey showed him pulling into a 3-point lead against President Obama on Saturday. All of the interviews in the Rasmussen poll were conducted after the convention began, although only about one-third were conducted after Mr. Romney's acceptance speech on Thursday night.

The Rasmussen poll represents a 5-point swing toward Mr. Romney from the polling firm's final survey before the conventions, when it had Mr. Obama ahe ad by two points. But it does not read quite as strongly for him as compared to the long-term average of Rasmussen polls, which have had Mr. Romney ahead by about one percentage point on average over the past 60 days.

Another survey, an online tracking poll conducted by the polling firm Ipsos, had previously shown a decent-sized bounce for Mr. Romney - but it has since receded some. In the version of the poll that Ipsos released on Saturday, Mr. Obama led by one percentage point, 44-43, among likely voters. That's a better result for Mr. Romney than the survey the firm conducted prior to the conventions, when Mr. Romney had trailed by four points. But it reflects a reversal from Thursday, when Mr. Romney was up by two points in the poll.

The most sluggish of the tracking polls is from Gallup, which reports its results over a lengthy seven-day window. That means that only about half its interviews occurred after the start of the c onvention, and a smaller fraction than that will represent people surveyed after Mr. Romney's acceptance address.

However, the trend so far in the Gallup poll is a bit disappointing for Mr. Romney; the survey still shows Mr. Obama one point ahead. By comparison, the Gallup poll has had a 46-46 tie on average over the past 60 days.

We'll need to wait another day or two before we can make a more confident judgement on the size of Mr. Romney's bounce, but the information we have so far points toward its being a little underwhelming.

The FiveThirtyEight “now-cast”, which does not adjust for the bounces associated with the party conventions, estimates that Mr. Obama would have a 72.3 percent chance of winning if the election were held today. That's essentially unchanged from before the conventions, when the number had ranged between about 70 percent and 74 percent.

One way to interpret the trend in the “now-cast” is that, so far, Mr. Romney's bounc e is hard to distinguish from the statistical noise that we ordinarily see in polls. Based on the data that they published on Saturday, Mr. Romney's standing in the Rasmussen poll was two points better than its 60-day average, but it was one point worse than average in the Gallup poll.

The FiveThirtyEight forecast, which penalizes a candidate in its evaluation of polls conducted just after his party convention, interprets the data as being slightly negative for Mr. Romney. On Saturday, Mr. Obama's chances of winning the Electoral College rose to 73.1 percent in the forecast, its highest figure since Aug. 16, when it was 73.6 percent.

But whatever assumptions you make about a convention bounce, it introduces more noise into the polling. Both the forecast and the “now-cast” are therefore likely to be more unstable than usual over the next week or two, and they should be interpreted with plenty of caution.

If Mr. Romney's bounce turns out to be mediocre, t he pressure will then be on Mr. Obama. If the polls fail to move much in his direction after the Democratic convention in Charlotte, N.C., the gains he has made in the FiveThirtyEight forecast over the past few days will be reversed.